The invention relates generally to integrated optical circuits and components making up such circuits, and, more particularly to low-loss, single-mode channel waveguides that operate at the 1.3 or 1.6 .mu.m wavelengths, or at longer wavelengths.
The recent development of low-loss single mode optical fibers with low dispersion at the 1.3 or 1.6 .mu.m wavelengths has focused attention on long-wave integrated optical circuits and optical systems that couple to such fibers. Such optical circuits and systems are useful in telecommunication, datacommunication, optical signal processing, optical interconnection, optical sensoring, and microwave antenna control applications. Semiconductor guided-wave circuits are of special interest because they could, in principle, provide optic-electronic integration; that is, the monolithic integration of optical guided-wave components with electronic circuits and with optical electrooptical components on a single "chip."
The fundamental building block of such guided-wave circuits is the channel waveguide which would be used to make directional couplers, optical switches, optical modulators, and optical interconnects between the various components. It is essential that optical-propagation losses be kept to a minimum in such channels (less than 1 dB/cm) to allow multiple guided-wave components to be cascaded on one wafer (such as a switching network) without incurring a significant loss penalty.
Another important need is to provide channel waveguides with physically small size so that the waveguides may be densely packed on a chip. In addition, the waveguide fabrication technique should be relatively simple, and should have the capability of permitting the fabrication of a wide variety of channel waveguide shapes. For example, the technique should allow stacked 3-dimensional integration of waveguide components as well as planar side-by-side (2 dimensional) integration of components.
Two prior art waveguide fabrication techniques have been used with some success, hetero-epitaxy of III-V semiconductors and homoepitaxy of silicon on silicon.
Prior art fabrication of III-V semiconductor guided-wave components for 1.3 to 1.6 .mu.m wavelengths suffers from the complexity of using binary, ternary, and quaternary alloy compositions of various materials, and the problems which arise from hetero-epitaxy of different volatile materials upon each other. As a result, these techniques are extremely sophisticated and do not always consistently produce good quality components. Also, since conventional hetero-epitaxy techniques are somewhat complex, they are relatively expensive.
In the past epitaxial silicon-on-silicon waveguides have been formed into channels by dry etching. In such a procedure, a lightly doped waveguiding layer is grown on a heavily doped substrate. The refractive index of the substrate is typically 0.01 lower than the index of the epitaxial guiding layer. The problem with such a prior art technique is that the guided-mode light tends to leak or "tail" evanescently into the heavily doped substrate because the index step is not large enough between the epitaxial layer and substrate to offer "tight" mode confinement. Moreover, the substrate tends to yield high optical losses due to the large concentration of free carriers therein. As a result, the optical propagation loss of channels fabricated with this homojunction prior art technique is in the range of 10 to 15 dB/cm. Losses in this range are not acceptable for medium-scale integration of guided-wave components. Also, it is impractical to shrink the waveguide core channel area down to 1 .mu.m or less because the optical losses become very large due to the extremely high doping of the Si waveguide "cladding."
It is clearly evident that there exists a need for improved optical channel waveguides which are not subject to the loss, quality and size drawbacks associated with prior art waveguides. A need also exists for improved techniques that simplify integrated optical chip manufacturing and consistently produce high quality hererostructure optical and electrooptical chips at low cost.